Setup Ts Deep Modules

# Setup TS Deep Modules

Make every package in this repo a **deep module**: a lot of behaviour behind a small interface. A package's public surface is its **entry points** (the files at the package root), and everything in its subfolders is hidden. This skill installs [dependency-cruiser](https://github.com/sverweij/dependency-cruiser) and the rules that make the entry points the only way in, then proves the rules bite.

For the vocabulary (deep module, interface, seam, depth), call the Skill tool with "codebase-design" and use its language throughout.

## The shape this enforces

```
src/packages/
  <name>/
    index.ts        ← an entry point (public). Import this from outside.
    client.ts       ← another entry point. Packages may expose SEVERAL.
    lib/            ← implementation: hidden from outside, free to import each other.
    tests/          ← co-located tests + fixtures (a subfolder, so private).
```

The public surface is the package's **root files**, not one designated `index.ts`. By convention implementation lives in `lib/` and tests in `tests/`, giving every package the same two-folder shape. The rule itself is general, though: *anything* in *any* subfolder is private, so you never extend the config to add a folder.

Four rules, all `error`:

1. **Entry-point boundary**: code outside a package (app code or another package) may import only that package's entry points (its root files), never anything in its subfolders.